Secure storage of a codeword within an integrated circuit

1. An integrated circuit comprising a plurality of transistors that store a codeword determined by a difference in mobility between a first transistor and a further transistor of the plurality of transistors, wherein the plurality of transistors are arranged to form a bi-stable circuit having two stable states; and wherein the difference in mobility between the first transistor and the further transistor biases the bi-stable circuit towards one or the other of the stable states to determine the codeword.

2. The integrated circuit of claim 1 , wherein the plurality of transistors comprise a first branch of series connected transistors comprising the first transistor, and a second branch of series connected transistors comprising the further transistor; wherein an output node of the first branch is connected to an input node of the second branch, and wherein an output node of the second branch is connected to an input node of the first branch, thereby forming the bi-stable circuit.

3. The integrated circuit of claim 1 , comprising a plurality of the bi-stable circuits.

4. The integrated circuit of claim 1 , wherein at least the first transistor has a width to length ratio of less than one.

5. The integrated circuit of claim 1 , wherein at least the first transistor is located at a predetermined location of the die of the integrated circuit, the predetermined location being more susceptible to undergo physical stress than other areas of the die.

6. The integrated circuit of claim 5 , wherein the predetermined location is at a corner or an edge of the die of the integrated circuit.

7. The integrated circuit of claim 1 wherein a stress layer (CAP) is provided to influence the stress within the transistors, thereby controlling the mobilities of the transistors and hence the value of the codeword.

8. The integrated circuit of claim 7 , wherein the stress layer comprises a compressive stress layer and a tensile stress layer, the compressive stress layer setting a first level of mobility and the tensile stress layer setting a second level of mobility.

9. Apparatus comprising the integrated circuit of claim 1 and a reader for connecting to the integrated circuit and reading the codeword, the reader comprising a driver for driving the integrated circuit to determine the codeword.

10. A method for reading a codeword stored in an integrated circuit according to claim 1 , the method comprising driving the integrated circuit to determine a codeword, the codeword being determined by the difference in mobility between the first transistor and a further transistor of a bi-stable circuit of the integrated circuit.